This invention relates generally to treating fluid production and subterranean formation treating methods, as well as compositions for treating fluid, particularly as they include at least part of previously used treating fluid. “Treating” is used as understood in the oil and gas industry. For example, this includes stimulation treatments. Examples of such treatments to which the present is particularly suited are fracturing and gravel packing. Much of the following explanation will be given with reference to fracturing, which is one limitation as to narrower aspects of the present invention; however, the overall explanation and broader aspects of the invention should be considered in the context of well stimulation or well treatment in general.
Producing subterranean formations penetrated by well bores are often treated to increase their permeabilities or conductivities. One production stimulation treatment involves fracturing the formation utilizing a viscous treating fluid. That is, the subterranean formation or producing zone is hydraulically fractured whereby one or more cracks or “fractures” are produced. Fracturing may be carried out in wells that are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing or other stimulation procedures are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations for the injection of fluids may also be fractured to facilitate the injection of the fluids.
Hydraulic fracturing is accomplished by injecting a viscous fracturing fluid into a subterranean formation or zone at a rate and pressure sufficient to cause the formation or zone to break down with the attendant production of one or more fractures. As the fracture is created, a portion of the fluid contained in the viscous fracturing fluid leaks off into the permeable formation and a filter cake comprised of deposited gelling agent is built up upon the walls of the fracture which then helps to prevent or reduce further fluid loss from the fracturing fluid to the formation. The continued pumping of the viscous fracturing fluid extends the fractures, and a proppant such as sand or other particulate material may be suspended in the fracturing fluid and introduced into the created fractures. The proppant material functions to prevent the formed fractures from closing upon reduction of the hydraulic pressure which was applied to create the fracture in the formation or zone whereby conductive channels remain through which produced fluids can readily flow to the well bore upon completion of the fracturing treatment.
The fracturing fluid must have a sufficiently high viscosity to retain the proppant material in suspension as the fracturing fluid flows into the created fractures. A viscosifier has heretofore often been utilized to gel a base fluid whereby a fracturing fluid having the high viscosity needed to realize the maximum benefits from the fracturing process is provided. After the high viscosity fracturing fluid has been pumped into the formation and fracturing of the formation has occurred, the fracturing fluid generally has been caused to revert into a low viscosity fluid for removal from the formation by breaking the gel. The breaking of viscosified fracturing fluids has commonly been accomplished by adding a breaker to the fracturing fluid prior to pumping it into the subterranean formation.
The fracturing fluids utilized heretofore have predominantly been water based liquids containing a gelling agent comprised of a polysaccharide such as guar gum. Guar and derivatized guar polymers such as hydroxypropylguar are economical water soluble polymers which can be used to create high viscosity in an aqueous fluid and are readily crosslinked which further increases the viscosity of the fluid. While the use of gelled and crosslinked polysaccharide fracturing fluids has been highly successful, the fracturing fluids have not been thermally stable at temperatures above about 200° F. That is, the highly viscous gelled and crosslinked fluids lose viscosity with time at high temperatures. To offset the loss of viscosity, the concentration of the gelling agent has been increased which involves increased cost and causes increased friction pressure in the tubing through which the fluid is injected into a subterranean formation which makes pumping of the fracturing fluids more difficult. Thermal stabilizers such as sodium thiosulfate have been included in the fracturing fluids to scavenge oxygen and thereby increase the stabilities of the fracturing fluids at high temperatures. However, the use of thermal stabilizers also increases the cost of the fracturing fluids.
Another situation which has been experienced in the use of gelled and crosslinked polysaccharide fracturing fluids involves the breaking of such fracturing fluids after fractures have been formed. Breakers such as oxidizers, enzymes and acid release agents that attack the acetal linkages in the polysaccharide polymer backbone have been used successfully.
In order to make the heretofore used gelled and crosslinked polysaccharide fracturing fluids carry sufficient proppant, the concentration of the crosslinking agent utilized has often had to be increased which in turn increases the cost and viscosity of the fracturing fluid. The water based fracturing fluids including gelled and crosslinked polysaccharide gelling agents have had significantly reduced fluid loss as compared to other fracturing fluids which reduces or eliminates the need for costly fluid loss additives. However, because the gelled and crosslinked polysaccharides have had high molecular weights, the filter cake produced from the viscous fracturing fluid on the walls of well bores penetrating producing formations and in fractures formed therein is often very difficult to remove.
In the use of a water based fracturing fluid including a gelled and crosslinked polysaccharide gelling agent, it has been mixed in holding tanks for a considerable length of time for hydration of the gelling agent to occur. During the fracturing process carried out in a well, the hydrated fracturing fluid generally is pumped out of the holding tanks, mixed with proppant and other additives on the fly, and pumped down the well bore to the formation being fractured. If during the job the downhole pressure profile and other parameters that are obtained in real time indicate that a change in the fracturing fluid properties is required, that is, a change in the fracturing fluid viscosity to prevent a screen out of the fracture or the like, for example, it is generally difficult or impossible to do so since it takes a very long time for a change to be made and for the changed fracturing fluid to reach the formation being fractured. Also related to pumping the fracturing fluid from holding tanks and combining the proppant material, crosslinker and other additives used on the fly is that the procedure requires the use of expensive metering and other similar equipment.
Also, in many environmentally sensitive areas, the water based fracturing fluids containing polysaccharide gelling agents must be recovered from the well and disposed of by environmentally appropriate means, which increases the overall cost of the fracturing treatment.
Thus, there have been needs for improved subterranean formation treating fluids and methods whereby the fluids are not thermally unstable, do not produce insoluble residues, have high proppant carrying capacities, produce easily removed filter cake, do not have to be hydrated in holding tanks for long periods of time, can have their properties changed during use, and can be recovered and reused if desired.
One or more of such needs are met by the invention described in U.S. Pat. No. 6,488,091, which invention provides subterranean formation treating fluid concentrates, improved treating fluids that can be utilized for fracturing as well as various other subterranean formation treatments, and methods of using the treating fluids.
A subterranean formation treating fluid concentrate of this prior invention is basically comprised of water and a depolymerized substantially fully hydrated polymer. The treating fluid concentrate can also include a variety of additives required to be in treating fluids produced utilizing the concentrate, such additives including pH adjusting compounds for adjusting the pH of the treating fluid formed with the concentrate, buffers, dispersants, surfactants for preventing the formation of emulsions between the treating fluid formed with the concentrate and subterranean formation fluids, bactericides and the like.
The treating fluid concentrate is prepared at a location away from the site of a well to be treated and transported to the well site prior to use. The concentrate is substantially fully hydrated and can be stored for long periods of time prior to its use. When used, the concentrate is continuously mixed with water and any additional additives required and pumped into the subterranean formation to be treated by way of the well bore penetrating it. Because there is very little time delay involved in mixing the treating fluid concentrate with additional water and other additives and pumping the treating fluid formed into a subterranean formation, the properties of the treating fluid can be periodically or continuously changed during the time that the pumping of the treating fluid takes place.
The improved subterranean formation treating fluids of such prior invention are basically comprised of water, a substantially fully hydrated depolymerized polymer, a pH adjusting compound for adjusting the pH of the treating fluid to an optimum level for crosslinking, and a crosslinking agent for crosslinking the substantially fully hydrated depolymerized polymer. While the improved subterranean formation treating fluids of this prior invention can be utilized for carrying out a variety of subterranean well treatments such as fracturing subterranean formations, forming gravel packs in subterranean formations, forming temporary blocking in the well bore, and as completion fluids and drill-in fluids, they are particularly useful as fracturing fluids for producing one or more fractures in a subterranean formation. When utilized as a fracturing fluid, the treating fluid generally contains a crosslinking agent and a proppant material which are mixed with the treating fluid when it is formed by mixing the treating fluid concentrate described above with additional water. The substantially fully hydrated depolymerized polymer utilized in the concentrate and the treating fluid produced therefrom is preferably a depolymerized polysaccharide polymer, and most preferably depolymerized hydroxypropylguar.
In accordance with a method of the prior invention, a gelled and crosslinked treating fluid is prepared comprised of water, a substantially fully hydrated depolymerized polymer, a pH adjusting compound for adjusting the pH of the treating fluid to an optimum pH for crosslinking, and a crosslinking agent for crosslinking the substantially fully hydrated depolymerized polymer. Thereafter, the gelled and crosslinked treating fluid is introduced into the subterranean formation to be treated. Particularly in the context of a fracturing treatment, the subterranean formation is contacted with the gelled and crosslinked fracturing fluid under conditions effective to create at least one fracture in the subterranean formation. After a fracture is created in the formation, a proppant material may be admixed with a portion of the fracturing fluid and introduced into the created fracture to ultimately prop the created fracture in an open position after the completion of the fracturing treatment.
One feature of the aforementioned prior invention is that the depolymerized polymer which is crosslinked to increase the viscosity of the treating fluid as desired can be delinked (uncrosslinked) and later relinked, whereby at least this part of the prior treating fluid can be reused in a later-prepared treating fluid. This may reduce cost of treating fluid itself as well as of disposal of it, and it may reduce exposure to volatile markets or supplies of fluid constituents and to quality assurance issues. Cost reduction due to reuseability of part of a prior treating fluid can make otherwise more expensive treating fluids accessible to more wells.
Although the aforementioned prior invention satisfies one or more of these needs, there are additional needs, such as how to accomplish this reusability and what type of composition to provide for reuse. Particular needs include how to select and maintain a reusable portion of a prior fracturing fluid and how to prepare it for reuse. Another need includes pricing for such reuse.